Storm Forecast
Valid: Thu 13 May 2010 06:00 to Fri 14 May 2010 06:00 UTC
Issued: Wed 12 May 2010 22:26
Forecaster: GATZEN

A level 1 was issued for Romania and northern Bulgaria mainly for large hail and tornadoes as well as excessive rain.

A level 1 was issued for the southern Balkans mainly for large hail.

A level 1 was issued for Iberia and northern Italy mainly for large hail.

SYNOPSIS

An unseasonably intense cold air outbreak continues across western Europe. A broad amplified trough stretches from northern Morocco to France and the Alps, supporting a very strong westerly jet to the south with winds up to more than 45 m/s over Morocco and Algeria. While the main trough axis starts to turn south-eastwards into Africa, the flow turns more south-westward over the Mediterranean and south-eastern Europe. For eastern Europe, a rather weak mid-level flow will remain. At lower levels, polar air masses dominate the western portions, while rather warm air across eastern Europe spreads into Scandinavia. Over south-eastern Europe a tongue of hot desert air is advected into the Aegean.

DISCUSSION

The Balkans

A frontal boundary has developed over the Balkans at the eastern flank of the European trough. On Thursday, a weak surface trough is expected to travel across the Adriatic into the Balkans that supports some warm air advection from Greece into Bulgaria. This will be associated with another elevated mixed layer spreading northward. The boundary-layer moisture will decrease to the south of the surface front that is expected across central Bulgaria during the day, while a weak easterly flow associated with rich boundary-layer moisture is forecast over Romania.

Given the steep lapse rates and some boundary-layer moisture, instability will likely develop over a broad region. Given the weaker low-level moisture and low-level convergence, chance of initiation is limited in the southern portions. Storms that manage to evolve will likely profit from moderate deep layer vertical wind shear of 15 m/s, and the combination of dry low-level air and steep lapse rates may increase the chance of large hail and downbursts. The convection is expected to decay in the evening hours. To the north, low-level convergence and ample moisture are forecast to increase the chance of thunderstorms significantly. While the backed low-level winds will be supportive for rotating updrafts, the weaker mid-level flow and deep layer vertical wind shear below 15 m/s are forecast to limit the potential of long-lived storms. Along the frontal boundary, a couple of intense cells are expected nevertheless that will likely evolve rotating updrafts. Storms that form may produce tornadoes, especially in the vicinity of outflow boundaries of earlier convection. The main threat will be large hail, though. Local flash flooding due to the movement of the storms along the frontal boundary is not ruled out, where storms are expected to cluster and will likely continue through the night.

Eastern Europe

Weak vertical wind shear (10 m/s deep layer) and quite well-mixed low-level air coupled with adequate moisture seems to support pulse storms during the day that develop along of outflow boundaries. These storms are forecast to produce isolated large hail and severe wind gusts as well as local flash flooding due to their slow movement. Additionally, tornadoes are not ruled out given the good low-level buoyancy and possibly favourable low-level hodographs near to outflow boundaries. Storms will likely decay after sunset over most portions, while stronger low-level convergence may support some nocturnal convective systems over Poland east of the main cold front.

Iberia to southern France and northern Italy

The polar air mass that is present over western Europe has become unstable on Wedensday due to low-level heating, especially across Italy, where the low-level moisture is rather rich. On Thursday, diurnal heating and some mid-level forcing ahead of the approaching jet axis will likely lead to steepening lapse rates, and given some low-level moisture, instability is expected to develop. Latest models indicate weak instability over France due to cold Mistral winds, and the best potential of thunderstorms seems to exist across Italy. While the low-level hodographs will be weak due to the cold air advection, some deep layer vertical wind shear is expected across most of the region that may support multicells capable of producing large hail. The slow moving storms may also produce local flash flooding.